Complications arising from reduction of blood supply to the heart are a leading cause of death and debilitation worldwide. Nevertheless, restoration of coronary blood flow to seemingly viable myocardial tissue is often accompanied by loss of cardiac function and, in the long term, development of heart failure. This paradoxical phenomenon, broadly termed ischemia/reperfusion injury, is manifested more severely in the elderly. Mitochondria likely play a central role in myocardial ischemia/reperfusion injury. Critical for the maintenance of cardiac energy status and function, mitochondria exhibit declines in the rate of respiration and oxidative phosphorylation during ischemia, with further age-dependent deficits evident upon reperfusion. The proposed studies seek to define mechanisms responsible for this loss in function by unifying two events associated with ischemia/reperfusion: Ca2+ overload and pro-oxidant production. Cardiac ischemia results in detachment of cytochrome c from the inner mitochondrial membrane, an event responsible for declines in the rate of electron transport. During reperfusion, the redox sensitive enzymes complex I, a-ketoglutarate dehydrogenase, and aconitase exhibit declines in activity. Exposure of isolated mitochondria to alterations in pH and Ca2+ concentration that mimic the transition from ischemia to reperfusion results in cytochrome c dissociation and oxidative inhibition of these redox sensitive enzymes. Depending on the magnitude and duration of oxidative stress, reversible inhibition can progress to irreversible inactivation. It is hypothesized that: Increases in mitochondrial Ca2+ concentration during myocardial ischemia/reperfusion lead to dissociation of cytochrome c from the inner mitochondrial membrane resulting in an increase in free radical production and oxidative inhibition of redox sensitive enzymes. This transiently reduces the rates of mitochondrial respiration, free radical production, and susceptibility to irreversible oxidative damage. Aging augments mitochondrial Ca2+ overload, increasing the likelihood of progression from reversible modulation to irreversible inactivation of mitochondrial function. Utilizing an in vivo rat model, durations of cardiac ischemia and reperfusion will be varied using animals of different ages to identify molecular events that result in increased mitochondrial pro-oxidant production (Aim 1), specific targets and mechanisms of redox- dependent modification (Aim 2), biochemical consequences of oxidative modification (Aim 2), and age- dependent factors that promote irreparable loss in mitochondrial function during cardiac ischemia/reperfusion (Aims 1 and 2). Elucidation of molecular events responsible for ischemia/reperfusion injury is required for optimization of strategies for favorably influencing the outcome particularly in the elderly population. Lay Description: Heart disease is a leading cause of debilitation and death, particularly in the aging population. Our studies seek to define age-related factors that enhance the severity of heart disease in an effort to design strategies to improve the outcome.